The design and creation of ionic crystals that show high mobility of ionic species in the solid state has long been a research topic of considerable attention not only due to the practical applications of these materials but also due to the correlation of such ionic species with ion-transport biological systems. In this work, we report the mobility of alkali metal ions (M = Li⁺, Na⁺, K⁺) in the ionic crystals M₆[Rh₄Zn₄O (l-cysteinate)₁₂]·nH₂O (M₆[1]·nH₂O). In M₆[1]·nH₂O, alkali metal ions are distributed in a disordered manner, together with a number of water molecules, within a rigid hydrogen-bonded framework of anionic clusters of [1]^6-. The alternating current conductivities of M₆[1]·nH₂O in the solid state increase in the order of Li₆[1]·nH₂O < Na₆[1]·nH₂O < K₆[1]·nH₂O, which is opposite to the order of the naked ionic radii. The conductivities reach the superionic level of σ = 1.3 × 10^-2 S cm^-1 at 300 K for a single crystal of K₆[1]·nH₂O. These results reflect the high mobility of hydrated alkali metal ions in the crystal lattice of M₆[1]·nH₂O, which is supported by solid-state NMR spectroscopy, together with ion diffusion experiments in the solid state. The high mobility leads to quick exchange of K⁺ ions in K₆[1]·nH₂O with Li⁺ and Na⁺ ions with retention of single crystallinity.

Yosuke Fukuda

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